When an eye's optical power is perfectly matched to its length, the eye is said to be emmetropic. Emmetropia is the medical term for 20/20 vision needing no corrective lenses, contact lenses, or reading glasses. It occurs because the optical power of the eye can perfectly focus an image to the retina, giving them "perfect" vision.
The opposite of emmetropia is ametropia. With ametropia, the focal point of the eye is some distance in front of or behind the retina.